1. Field of the Disclosure
The present disclosure relates to a fuel reformer, and particularly, to a method for controlling a fuel reformer, which is capable of allowing a catalyst of the fuel reformer to reach a starting temperature within a shortest time.
2. Background of the Disclosure
A fuel cell indicates an energy conversion apparatus capable of directly converting chemical energy into electric energy, by a chemical reaction between fuel (hydrogen) and an oxidant (oxygen), through a reverse reaction of a water electrolysis. Hydrogen gas supplied to an anode of the fuel cell is divided into hydrogen ions and electrons through a catalyst. Then, the hydrogen ions move to a cathode of the fuel cell through an electrolyte, and the electrons move to the cathode through an external circuit. Oxygen gas supplied to the cathode is dissociated into oxygen atoms through a catalyst. Then, the oxygen atoms react with the hydrogen ions which have moved through the electrolyte, and the electrons which have moved through the external circuit, thereby generating water.
According to a type and an operation principle of an electrolyte, the fuel cells may be largely categorized into alkali fuel cells, phosphoric acid fuel cells, polymer electrolyte membrane fuel cells (PEMFC), direct methanol fuel cells, molten carbonate fuel cells (MCFC), and sold oxide fuel cells (SOFC). In case of the PEMFC, a polymer membrane such as nafion is used as an electrolyte, and the polymer electrolyte membrane fuel cell smoothly operates when the electrolyte contains a large amount of water. Therefore, hydrogen and oxygen are supplied to a fuel cell stack in a humidified state so that the relative humidity is about 100%. The polymer electrolyte membrane is provided with water from gas supplied to a reactant (reaction material).
Most of fuel cells including polymer electrolyte membrane fuel cells use hydrogen as fuel. However, there is a limitation in adopting fuel cells using hydrogen in the current situation where hydrogen supply is not sufficient. Accordingly, required is a transitional system capable of using a fuel cell after generating hydrogen by reforming hydrocarbon fuels such as natural gas, ethanol, gasoline and diesel. A fuel cell system being currently developed is mounted with a fuel reformer due to such reasons.
A driving method of the fuel reformer may include steam reforming, partial oxidation reforming and auto-thermal reforming. The steam reforming is a method for acquiring hydrogen through an endothermic (heat absorption) reaction between fuel and steam. The steam reforming is advantageous in that a yield of hydrogen is excellent. However, such method is disadvantageous in that heat should be continuously supplied from outside. The partial oxidation reforming is a method for acquiring hydrogen through an exothermic reaction between gasoline fuel and oxygen. In case of the partial oxidation reforming, a yield of hydrogen is lower than in case of the steam reforming. However, the partial oxidation reforming is advantageous in that additional heat supply from outside is not required. The auto-thermal reforming is advantageous in that a reactor can be designed more effectively as both of the steam reforming and the partial oxidation reforming are used. Accordingly, in case of a fuel cell system which is driven in an independent manner, it is advantageous to adopt an auto-thermal reforming method capable of rapidly performing a starting operation and requiring no additional heat supply from outside.
After a catalyst of an auto-thermal reforming (ATR) reactor reacts with hydrocarbon fuels, the hydrocarbon fuels are decomposed to generate a gas group containing a large amount of hydrogen. Generally, the generated gas consists of hydrogen, carbon monoxide, carbon dioxide, nitrogen, steam, etc. Among such gases, the carbon monoxide causes a polymer electrolyte membrane fuel cell (PEMFC) not to normally operate, by poisoning a platinum catalyst of the PEMFC. Therefore, carbon monoxide generated after a reaction should be removed to 10 ppm through an additional reactor. Generally, a fuel reformer reduces the amount of carbon monoxide to 1% using a water gas shift (WGS) reactor, and reduces the carbon monoxide to 10 ppm through a preferential oxidation reaction (PROX or selective catalytic oxidation). Considering such features, most of fuel cells are provided with an auto-thermal reforming reactor (which can be replaced by a steam reforming reactor and a partial oxidation reforming reactor in some cases), a WGS reactor and a PROX reactor.
There are two types of catalysts used in a fuel reformer. A pellet type catalyst is implemented in the form of grains of ceramic. On the other hand, a monolith type catalyst is implemented coated on cells of a ceramic supporter. The pellet type catalyst is mainly used in a home fuel cell system due to its low production cost. On the other hand, the monolith type catalyst is mainly used in a mobile fuel cell system due to its excellent environment-resistance (vibrations/impacts).
In order for an auto-thermal reforming (ATR) reactor to start a fuel reforming reaction, temperature of a catalyst should be increased up to about 200˜300° C. This is called a starting temperature of a catalyst. Such starting temperature may be variable according to a fuel type. Once the temperature of the catalyst of the ATR reactor is increased up to 200˜300° C., reactants are supplied to the catalyst to induce an exothermic reaction. Once the ATR reactor starts to perform an exothermic reaction, a larger amount of heat may be generated than in a case using an electric heater. By using such gas of high temperature, a WGS reactor and a PROX reactor may be heated up to a target temperature. For rapid start-up, a fuel reformer should be designed so that temperature of a catalyst of an auto-thermal reforming reactor can be quickly increased, and then an exothermic reaction can be performed.
The most general method for increasing a catalyst temperature is using heating wires mounted to outside of a case of a reactor. In case of a pellet type catalyst, a heat transfer coefficient is small because a main material of the catalyst is ceramic. Accordingly, it takes a lot of time for heat supplied from external heating wires to be transferred up to inside of the laminated pellet type catalyst. In case of a monolith type catalyst, it takes less time for heat supplied from external heating wires to be transferred up to inside of the catalyst, than in the case of the pellet type catalyst. However, it is difficult to anticipate rapid heat transfer, because an insulator is disposed between the monolith catalyst and the case in order to fix the monolith catalyst into the case.
Another method for increasing temperature of a catalyst inside a reactor is to insert heating wires into a region near the catalyst mounted to inside of a case of the reactor. In this case, time taken for a catalyst temperature to be increased can be more reduced than in the case using the aforementioned method. However, the inner structure of the reactor may become complicated and the fabrication costs may be increased, because the heating wires should be inserted into the reactor through the case. Further, in case of using the heating wires, an excessive amount of electric energy is required.
A method for heating a catalyst within the shortest time is to heat the catalyst by combustion. A fuel cell system is provided with fuel for driving a reactor. Accordingly, a catalyst temperature can be easily increased by merely adding an apparatus for combusting the fuel. Generally, combustion heat has thermal energy much higher than that generated from an electric heater, so that a catalyst can be heated more rapidly. Fuel reformers which have been developed so far have a structure that a combustion space is provided at an upper end of a catalyst, and the catalyst undergoes temperature rising by combustion gas or combustion flame of high temperature. To this end, an ignition plug for igniting fuel is positioned at an upper end of the catalyst. More specifically, flame combustion occurs from the upper end of the catalyst, thereby rapidly increasing temperature of the catalyst. However, such method has the following problems. Firstly, the catalyst may be drastically thermally-deteriorated due to thermal impacts. Further, soot may be absorbed to the surface of the catalyst when incomplete combustion occurs. This may degrade performance of the catalyst.